From 402e3142a94ebc438b3da693178c98c29b6f1a53 Mon Sep 17 00:00:00 2001
From: Karim El-Husseiny <karim.husseiny@gmail.com>
Date: Mon, 9 Feb 2026 16:38:51 +0200
Subject: [PATCH 1/2] fix: Pass metric_prefix to Aggregator finalizer

The Finalizer struct expects 6 arguments but only 5 were passed,
leaving metric_prefix as nil. This caused metrics to lose their
prefix when flushed during garbage collection.

The bug only manifests when:
1. Aggregator is created with a prefix
2. Metrics are added without tags (tags trigger datagram_builder creation)
3. GC runs before normal flush, triggering the finalizer
---
 CHANGELOG.md                        |  1 +
 lib/statsd/instrument/aggregator.rb |  1 +
 test/aggregator_test.rb             | 25 +++++++++++++++++++++++++
 3 files changed, 27 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index dbf3d20a..470b0cac 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@ section below.
 
 ## Unreleased changes
 
+- [#416](https://github.com/Shopify/statsd-instrument/pull/416) - Fix missing `metric_prefix` in Aggregator finalizer, causing metrics to lose their prefix when flushed during GC.
 - [#411](https://github.com/Shopify/statsd-instrument/pull/403) - Add `CompiledMetric::Gauge` as the second metric type to support pre-compiled metric datagrams. It can be used as a replacement over standard `StatsD.gauge`.
 - Add support for `nil` tag values in `CompiledMetric` dynamic tags (converted to empty string).
 - [#407](https://github.com/Shopify/statsd-instrument/pull/407) - Add support for `Symbol` and `:Boolean` types in `CompiledMetric` dynamic tags, allowing symbol and boolean values to be used as tag values.
diff --git a/lib/statsd/instrument/aggregator.rb b/lib/statsd/instrument/aggregator.rb
index f32f6104..f36d1b0e 100644
--- a/lib/statsd/instrument/aggregator.rb
+++ b/lib/statsd/instrument/aggregator.rb
@@ -132,6 +132,7 @@ def initialize(
           @datagram_builders,
           @datagram_builder_class,
           @default_tags,
+          @metric_prefix,
         )
 
         ObjectSpace.define_finalizer(
diff --git a/test/aggregator_test.rb b/test/aggregator_test.rb
index 6745330d..555bb301 100644
--- a/test/aggregator_test.rb
+++ b/test/aggregator_test.rb
@@ -179,6 +179,31 @@ def test_with_prefix
     assert_equal(1, @sink.datagrams.last.value)
   end
 
+  def test_finalizer_with_prefix
+    # Test that the finalizer correctly uses the prefix when flushing metrics
+    # IMPORTANT: Use empty tags to ensure datagram_builder is not pre-created by tags_sorted
+    aggregator = StatsD::Instrument::Aggregator.new(@sink, StatsD::Instrument::DatagramBuilder, "MyApp", [])
+
+    aggregator.increment("foo", 1, tags: [])
+    aggregator.increment("bar", 1, tags: [], no_prefix: true)
+
+    # Manually trigger the finalizer (simulates GC cleanup)
+    finalizer = StatsD::Instrument::Aggregator.finalize(
+      aggregator.instance_variable_get(:@finalizer),
+    )
+    finalizer.call
+
+    assert_equal(2, @sink.datagrams.size)
+
+    prefixed_datagram = @sink.datagrams.find { |d| d.name == "MyApp.foo" }
+    refute_nil(prefixed_datagram, "Expected to find datagram with prefix 'MyApp.foo'")
+    assert_equal(1, prefixed_datagram.value)
+
+    unprefixed_datagram = @sink.datagrams.find { |d| d.name == "bar" }
+    refute_nil(unprefixed_datagram, "Expected to find datagram without prefix 'bar'")
+    assert_equal(1, unprefixed_datagram.value)
+  end
+
   def test_synchronous_operation_on_thread_failure
     # Force thread_healthcheck to return false
     @subject.stubs(:thread_healthcheck).returns(false)

From cdc997d58c3a61eb42076dd981fd956b7598046a Mon Sep 17 00:00:00 2001
From: Karim El-Husseiny <karim.husseiny@gmail.com>
Date: Mon, 9 Feb 2026 16:44:21 +0200
Subject: [PATCH 2/2] Update CHANGELOG.md

---
 CHANGELOG.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8cdfd720..9f1d6cc0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@ creating a pull request, please add an entry to the "unreleased changes"
 section below.
 
 ## Unreleased changes
+
 - [#416](https://github.com/Shopify/statsd-instrument/pull/416) - Fix missing `metric_prefix` in Aggregator finalizer, causing metrics to lose their prefix when flushed during GC.
 - [#415](https://github.com/Shopify/statsd-instrument/pull/415) - Fix `sample_rate` being ignored when aggregation is enabled. Previously, `increment`, `measure`, and `histogram` calls would bypass sampling entirely when aggregation was enabled, causing metrics to be emitted at 100% rate regardless of the configured sample rate.
 - [#411](https://github.com/Shopify/statsd-instrument/pull/403) - Add `CompiledMetric::Gauge` as the second metric type to support pre-compiled metric datagrams. It can be used as a replacement over standard `StatsD.gauge`.